Technical Profile: Optimal pelleting

Very often when production capacity, or rather the capacity of the pellet press(es), starts to show shortcomings, the immediate action is to look for an additional pellet press line.

There is, however, an alternative — and less expensive — route.

OPTIMIZNG THE PELLET PRESS

Before considering investing in new press lines, it should be ensured that maximum throughput is gained from the presses already available.

This can be done by means of a patented system that has been on the market for several years, the Total Optimizing Pelleting System. This system from Norvidan Overseas A/S, Odense, Denmark, can, according to the company, increase the capacity of any given pellet press by 10% to 40%, avoiding the necessity to invest in more hardware.

The system consists of a PC control cubicle installed adjacent to the pellet press and a computer and software package installed in the plant control room. (See Figure 1). One PC/software package can control up to six press lines.

The basic principle of the optimizing system is to find the point for every single recipe/product where the pellet press tends to show gelatinization, as it has been well proven that the maximum throughput is just before the point of gelatinization.

The method of operation is illustrated in Figure 2. It can be described in the following way. From the PC, the press operator chooses the recipe to be produced. All settings at the system are stored in a database as well as dosing of any liquids in the conditioner/mixer.

The load of the press is gradually increased as the speed of the feeder screw is increased until 100% to 105% of the nominal load of the press is reached. The liquids, as well as steam, are dosed proportionally with the throughput during this load increase.

When the maximum press load point is reached, the steam addition is increased so that the meal will become easier to pelletize and the load on the press will go down for the same amount of meal throughput. This is compensated by increasing the speed of the feeder screw so the press load increases, which is again reduced by more steam.

The procedure through which the capacity is increased is continued until the gelatinization point is reached. Then, a remedy sequence is initiated to prevent the press from blocking. Several parameters are adjusted quickly and simultaneously, faster than even the most experienced press operator.

We now know where the gelatinization point or "disaster point" is where the press will block. Since the maximum capacity/throughput for any given recipe is just before blocking point, after the remedy sequence, the load/steam addition is brought back up to 95 percent of the gelatinization-blocking point.

The procedure as described above takes in total 2 to 3 minutes. With high frequency product/recipe changes, the system becomes even more efficient.

The main advantage of an optimization control system based on Norvidan’s patented "di/dt" function is the increased capacity.

There are, however, other positive results:

• The energy requirement per tonne of finished pellets is reduced by 10% to 30%.
• Die and roller wear are reduced by 10% to 30%.
• Press performance will become operator independent.
• Better logging and acquisition of production data.
• Better pellet quality.
• The system is able to accommodate any press line configuration.
• Includes facilities to automatically re-pellet if a given temperature has not been reached.

Additional process optimization

To further optimize pellet production, the next step would be to integrate on-line moisture measuring as well as an on-line pellet durability test with the pellet press optimization.

In this way, not only will the capacity be optimized, but also the moisture content and quality of the end product.

Norvidan Overseas A/S has for many years produced on-line moisture systems, which have a proven reliable measuring system (see Figures 3a and 3b), as well as on-line durability (see Figure 4) for feed plants. By integrating all three systems, a feed mill will have a system that automatically, and without any action of the operator, optimizes not only the capacity, but also durability and moisture content.